Phenotypic and Genotypic Determination of Metallo-Beta-Lactamases in Clinical Acinetobacter spp Isolates

Abstract

Carbapenem-resistant Acinetobacter isolates are increasingly isolated worldwide due to the intensive use of carbapenems. Metallo-beta-lactamases (MBL) are class B betalactamases and can hydrolyse all betalactams including carbapenems except aztreonam. It is necessary to detect carbapenem-resistant strains in routine microbiology laboratories to prevent the spread of antibiotic resistance and to choose the best treatment. Genotyping is reliable but expensive and complicated. For this reason, as an alternative to genotyping, cheap, reliable, and easily applicable phenotypic methods that can be used in daily practice are being developed. This study aimed to determine the presence of MBL causing carbapenem resistance in Acinetobacter isolates by genotypic and phenotypic methods for routine laboratory use. Double disc synergy test (DDST), combined disc diffusion test (CDDT), modified Hodge test (MHT) and carbapenemase inactivation method (CIM) were performed as phenotypic tests. Multiplex polymerase chain reaction (PCR) was performed to genotypically detect NDM, VIM, IMP, GIM, SIM, SPM gene regions responsible for MBL production. Among the phenotypic tests, the highest positivity rate was in the CIM test with 98 isolates (95.1%). CIM was followed by DDST with 94.1% and CDDT with 86.4%. The lowest positivity rate was found in MHT with 70.8%. NDM, VIM, IMP, SIM, SPM and GIM genes were not detected in the tested isolates. Despite high positivity in phenotypic tests, no positivity was detected in genotypic tests. There may be some reasons for this situation: Variants of these genes may not have been detected with the primers used. Due to the bactericidal effect of EDTA, false positivity may be seen in DDST and CDDT. Carbapenemases other than MBL, penicillin binding protein changes, efflux pumps, porin loss, and decreased permeability may have caused higher positivity in MHT and CIM. In our study, it was concluded that phenotypic tests significantly detected carbapenemase production, but should be used more carefully for the detection of MBL production.